Contact lenses are widely used for correcting many different types of vision deficiencies. These include defects such as near-sightedness and far-sightedness (myopia and hypermetropia, respectively), astigmatism vision errors, and defects in near range vision usually associated with aging (presbyopia).
Astigmatism is optical power meridian-dependent refractive error in an eye. This is usually due to one or more refractive surfaces, most commonly the anterior cornea, having a toroidal shape. It may also be due to one or more surfaces being transversely displaced or tilted. Astigmatism is usually regular, which means that the principal (maximum and minimum power) meridians are perpendicular to each other. People with astigmatism have blurred vision at all distances, although this may be worse at distance or near, depending on the type of astigmatism. These people may complain of sore eyes and headaches associated with demanding visual tasks. Astigmatism can be corrected with a toric contact lens, which usually has one spherical surface and one toroidal (cylindrical) surface which can be formed in either the posterior surface (back surface) or in the anterior surface (front surface) of the toric lens. Since astigmatism requiring vision correction is usually associated with other refractive abnormalities, such as myopia (nearsightedness) or hypermetropia (farsightedness), toric contact lenses are generally prescribed also with a spherical power correction to correct myopic astigmatism or hypermetropic astigmatism.
A conventional toric contact lens with one toroidal surface and one spherical surface typically will have an uncontrolled optical power profile. The optical power at any given position of the toric lens depends not only upon the distance from the optical axis (or lens center) but also upon angular position relative to the principal meridians of the toric lens. In addition, the optical power profile of a toric lens is dependent upon its targeted optical powers (i.e, Rx). The optical power of a toric lens can be composed of purely positive power deviations (i.e., the power at a position depart from the lens center being larger than the power at the lens center), purely negative power deviations (i.e., the power at a position depart from the lens center being less than the power at the lens center), or the combination of both, depending on the spherical optical power and the cylindrical optical power of the lens. With such uncontrolled optical power profile, a toric lens may not provide optimal vision to a patient, especially with a larger pupils.
Moreover, the spherical aberration can be an inherent high order aberration component of an eye. The spherical aberration generally is a rotationally symmetric aberration around the optical axis. A typical adult human eye, as a result of the optical characteristics of the cornea and crystal lens, inherently exhibits an increasing amount of spherical aberration (positive spherical aberration) as the diameter of the pupil expands. Typically, the spherical aberration, of an adult, is about one diopter at a 6 mm diameter pupil, while the spherical aberration is slightly less than two diopters at an 8 mm pupil. A toric lens with purely positive power deviation may not compensate but instead accentuate the inherent spherical aberration of an eye and as such, may not be able to provide a good vision to a patient with a relative large pupil or under a dark illumination condition (i.e., with a dilated pupil).
Therefore, it is advantageous that a toric lens is designed to have a controlled optical power profile, which is preferably capable of compensating the inherent spherical aberration of a typical human eye, so as to provide an improved vision to a patient.
An object of the invention is to provide a toric contact lens having a controlled optical power profile.
Another object of the invention is to provide a method for producing a toric contact lens having a controlled optical power profile.
A further object of the invention is to provide a family of contact lenses having a series of different cylindrical powers and a series of different spherical powers. Each lens in the series has a controlled optical power profile.